Video Transcript
Molecules of polar protic solvents
can form hydrogen bonds because they usually contain at least one hydrogen atom that
is covalently bonded to a nitrogen, oxygen, or fluorine atom. Which of the following substances
can be classified as a polar protic solvent? (A) Methyl sulfoxide, (B) ethanol,
(C) acetone, (D) dichloromethane, or (E) dimethylformamide.
A polar solvent is a solvent that
consists of polar molecules. Polar molecules have an
asymmetrical distribution of electron density due to polar bonds in the
molecule.
We can classify polar solvents into
two further classes: polar protic and polar aprotic solvents. The molecules of a polar protic
solvent are capable of forming hydrogen bonds. The molecules must have at least
one hydrogen atom covalently bonded to a nitrogen, oxygen, or fluorine atom. The hydrogen atom will have a
partial positive charge, which will allow strong intermolecular attractions to occur
with the electron lone pairs on electronegative atoms of adjacent molecules. While aprotic polar solvents would
still be composed of polar molecules, they would not possess the arrangement to form
hydrogen bonds.
In order to correctly identify the
polar protic solvent, let’s have a look at the molecular structures of our answer
choices.
Answer choice (A) is methyl
sulfoxide, often referred to as DMSO. Methyl sulfoxide has the chemical
formula (CH3)2SO and the displayed formula shown. If we look at the displayed
formula, we can see that the oxygen atom is not bonded to a hydrogen atom. The molecules of methyl sulfoxide
would not have the arrangement required to form hydrogen bonds with each other. And so answer choice (A) is not a
polar protic solvent.
Answer choice (B), ethanol has the
chemical formula C2H5OH. Its displayed formula is shown. The oxygen atom in this structure
is covalently bonded to a hydrogen atom. This creates the potential for
hydrogen bonds to form between molecules of ethanol, making it a polar protic
solvent. It is likely that answer choice (B)
is the correct answer to the question. But let’s have a look at the
remaining answer choices just to be sure.
The displayed formulas for acetone,
dichloromethane, and dimethylformamide are shown. Acetone does not have any hydrogen
atoms bonded to its oxygen. Dichloromethane does not have any
oxygen, nitrogen, or fluorine atoms at all. And dimethylformamide does not have
any hydrogen atoms bonded to its oxygen or nitrogen atoms. Answer choices (C), (D), and (E)
are not capable of hydrogen bonding and are thus not polar protic solvents.
Therefore, the substance that can
be classified as a polar protic solvent is answer choice (B), ethanol.
